1. Field of the Invention
The present invention relates to a machine tool, e.g. a grinding machine, a machining center or a lathe, and a bed thereof.
2. Description of the Related Art
It is well known for a machine tool, e.g. a grinding machine, a machining center or a lathe, that includes a work saddle, a tool box and a bed. The work saddle carries a workpiece. The tool box carries one or more machining tools. The bed consists of a work saddle-base to support the work saddle and a tool box-base to support the tool box. Such bed forms the base of the machine tool and is horizontally supported by suitable support members, e.g. jacks or leveling blocks, when the bed is fixed.
Generally, as shown in FIG. 1, prior art of the bed 10 of the machine tool is hollow and is made from a suitable material, e.g. a casting. In the box-like bed 10, plural reinforcement ribs 101 are disposed suitably to make the bed 10 be higher rigidity, considering the weight of the devices installed thereon. A grinding machine will be explained as an example of the well-known machine tool hereafter. The grinding machine comprises the tool box of a wheel head and the work saddle, not shown in FIG. 1. The wheel head advances and retracts on the tool box-base 12. The work saddle traverses the direction of the wheel head advancing/retracting on the work saddle-base 11, and carries a head stock and a tail stock chucking a workpiece therebetween. The points A1, A2 and A3 respectively represent the center of gravity of the wheel head, the head stock and the tail stock and move along the arrows shown in FIG. 1.
A ceiling, not shown, is fixed on the bed 10 of the grinding machine. To install the wheel head and the work saddle with the head stock and the tail stock on the ceiling of the bed 10, the hollow bed 10 is latticed by plurality of the reinforcement ribs 101 arranged in two directions crossing at a right angle therein. Then, at support points of the bed 10, there are arranged the support members under a floor 102 of the bed 10. The support members level the bed 10 and adjust static displacement (strain) of the bed 10, and further adjust dynamic displacement of the grinding machine that will be generated during grinding. In the prior art, for example, the number of the support members is thirteen.
In the above-mentioned machine tool, however, since the support members receive the load of the machine tool, the reaction force occurs against the load at each of the support points of the bed 10. Therefore, the deflection of the bed 10 is the largest around each of the support points and becomes gradually smaller as each of the support points goes away. On the other hand, as each of the support points goes away, the strain of the bed 10 becomes larger. Namely, around the support points, the increase of the strain is large but the amount of the strain is small. In contrast, as each of the support points goes away, the increase of the strain becomes smaller but the amount of the strain becomes larger. The strain of the ceiling, an upper part of the bed 10, has to satisfy the tolerance zone of the strain when the machine tool is fixed, thereby the bed 10 is supported under the floor 102 by plurality of the support members to make its strain become smaller. To support the bed 10 of the machine tool of the prior art by plurality of the support members, the height of each of the support members has to be respectively adjusted when the bed 10 is fixed on the floor. Thus, fixing the bed 10 is complicated and spends much time.
Further in the prior art, although the bed 10 is latticed by plurality of the reinforcement ribs 101 in itself, such ribs 101 may not maintain enough rigidity so that plenty of the support members are also needed when the bed 10 is fixed. Particularly, in the case that one support point is not located just below the reinforcement ribs 101, the support member thereat occurs large strain of the bed 10. Or, although one support point is located just below one of the reinforcement ribs 101, in the case that the reinforcement rib 101 is only for the rigidity of the bed 10 and does not support the ceiling, the support member thereat also occurs large strain of the bed 10. Nobody pays attention to the above-mentioned problem that plenty of the support members are used in order to make the strain of the bed 10 become smaller and satisfy the tolerance zone thereof.
As described above, the bed 10 of the prior art is supported and leveled by plenty of the support members adjusted respectively, thereby leveling the bed 10 requires great skill and spends time. Further, regular maintenance to level the bed 10 is required in order to compensate the aged deterioration. Furthermore, considering and bearing the weight of the wheel head, the head stock and the tail stock, the bed 10 forms T-like and box-like exterior, and forms plurality of the reinforcement ribs 101 therein, whereby the shape of the bed 10 and its die for molding are complicated. Because of the complication, the strain of whole bed 10 is also complicated. Therefore, the bed 10 may be costly in its production and may be unsatisfactory in its working accuracy.
Now, it is known that stuff can be leveled when supported by three support points. So, it may be considered that the bed 10 of the machine tool is also supported by three support points so as to reduce the number of the support points, thereby it is expected that fixing the bed 10 will be simplified and its time will be shortened. However, the bed 10 is unstably supported by means of simple support at three points, because the center of gravity of the machine tool moves according to the movable tool box or/and work saddle thereon. In such machine tool, the tool box and the work saddle relatively move on the bed 10 in order to machine the workpiece, so that the center of gravity of whole machine tool may go out of the triangle formed by three support points and the machine tool becomes unstable. That particularly happens to the machine tool of which center of gravity is widely moved by movement of the device(s), the tool box or/and the work saddle, on the bed 10, especially the device(s) being heavy, e.g. tool-box/work saddle with a motor thereon or plural tool boxes/work saddles. That is why the bed 10 supported by three points is not used for the prior art of the machine tool shown in FIG. 1.